Collaborative Research: Identifying the Limitations and Expanding the Utility of Detrital Mineral U-Pb Geochronology to Tectonic Studies

利用碎屑矿物（特别是锆石）的放射性年龄的物源研究在过去十年中得到了极大的扩展，这主要是由于微地质年代学方法的进步。正是这些特性（极高的闭合温度和极低的溶解度）使得锆石在碰撞造山过程中几乎是惰性的，除了上部角闪岩到麻粒岩相和深熔岩。因此，碰撞、地壳增厚、加热和加载、挖掘等整个周期可能在某些地质体中经过，而使用常规的碎屑锆石分析方法在碎屑记录中不会有该周期的记录。该研究项目正在测试确定碎屑独居石的铀-钍-铅年龄的效用，与仅分析碎屑锆石岩心相比，它具有提供更丰富的构造变质事件历史的潜力。此外，极小的碎屑锆石晶体和碎屑锆石上的薄变质边缘的假设，与通过分析较大颗粒的中心部分得到的结论相比，提供了一个实质上不同的造山带年代观，这一假设正在进行测试。来自南部阿巴拉契亚山脉的碎屑沉积岩和冲积层正在被取样，以寻找它们的重矿物层。之所以选择阿巴拉契亚造山带，是因为该带经历了全方位的构造过程，而不是主要由弧岩浆作用等单一过程构成。此外，为了充分验证这些假设，有必要从岩浆、变质和变形历史的细节已经确定的地区收集碎屑数据。这将允许建立最稳健的方案，以最大限度地减少年龄偏差的影响，年龄偏差既包括自然固有的碎屑矿物，也包括在谷物选择过程中潜在的实验室诱导偏差。只有从一个研究充分的多变形造山带获得碎屑年龄信息的结果，它们才能最好地应用于对其他更不了解的造山带的构造研究。这项研究的结果将对全球碎屑矿物年龄记录的解释产生重大影响。这里开发的最成功的方法和工具，可以解决目前存在的传统碎屑锆石数据库解释的问题，然后可供其他研究人员在任何造山带、任何地质时期的工作中使用，以最好地利用碎屑矿物定年作为物源指标，作为板块重建的辅助，并更准确地说明主要的大地构造-热事件。

Provenance studies utilizing radiometric ages of detrital minerals (particularly zircon) have expanded greatly over the past decade, driven largely by advances in microgeochronometric methods. The very same properties that make zircon such a powerful geochronometer (extremely high closure temperature and very low solubility) render it virtually inert during collisional orogenesis, except at upper amphibolite to granulite facies and anatexis. Thus, an entire cycle of collision, crustal thickening, heating and loading, and exhumation could pass in some terranes and there would be no record of that cycle in the detrital record using conventional approaches for detrital zircon analysis. This research project is testing the utility of determining the Uranium-Thorium-Lead ages of detrital monazite, which holds the potential for providing a far richer history of tectonometamorphic events compared to analyzing only detrital zircon cores. Additionally, the hypothesis that very small detrital zircon crystals, and thin metamorphic rims on detrital zircon, provide a substantially different chronological view of an orogen compared to that obtained by analyzing the central portions of larger grains is being tested. Clastic sedimentary rocks and alluvium from the southern Appalachians are being sampled for their heavy mineral suites. The Appalachian orogen is chosen because this belt has experienced the full spectrum of tectonic processes, rather than being constructed largely by a single process such as arc magmatism. In addition, to fully test these hypotheses it is necessary to collect the detrital data from a region where the details of the magmatic, metamorphic, and deformational history are well established. This will allow establishment of the most robust protocols to minimize the effects of age bias, both those naturally inherent to detrital minerals, and potential laboratory induced bias during grain selection. Only once the results of detrital age information are available from a well-studied, polydeformed orogen can they be best applied to tectonic studies of other more poorly understood orogenic belts. The results of this study will have major implications for the interpretation of the worldwide detrital mineral age record. The procedures and tools developed here that are most successful for resolving the problems that currently exist with the interpretation of the conventional detrital zircon database will then be available for application by other researchers working in any orogenic belt, for any division of geologic time, to best use detrital mineral dating as a provenance indicator, as an aid in plate reconstructions, and for much more accurate elucidation of major terrane tectonothermal events.